This invention relates to an improvement in the process whereby coal is catalytically hydrogenated to liquid products. More particularly, it relates to a process whereby a metal-containing hydrogenation catalyst is recovered from a hydrogenated coal slurry in an active form suitable for recycle to the process.
A number of processes have been developed in recent years for hydrogenating coal, usually bituminous or sub-bituminous coal, to gaseous and liquid products. Both catalytic and noncatalytic hydrogenation processes are known. The presently more important catalytic processes fall generally into two classes, those wherein a stationary bed of pelleted or extruded catalyst is used and the so-called ebullated bed type of process wherein the coal-oil slurry feed and hydrogen flow upwardly through a bed of relatively fine particles of catalyst, thereby imparting a random turbulence or boiling movement to the catalyst particles to improve contact between the catalyst and the mixed feed. The catalysts used in both types of process are usually supported metal-containing catalysts such as cobalt molybdate on alumina, molybdenum oxide or sulfide on an aluminum silicate support, and the like.
Another type of catalytic coal hydrogenation process which has had less attention in recent years is that where a catalyst is added directly to the coal-oil slurry, usually as a decomposable metal salt, where it remains dispersed in the reaction mixture. This kind of process is illustrated by Pelipetz, U.S. Pat. No. 2,860,101 which describes the addition of molybdenum catalyst by the impregnation of the coal with a water-soluble molybdenum salt. The recovery of catalyst from the product of this kind of process is obviously more difficult than the recovery of spent catalyst from ebullated bed or stationary bed processes where the bulk of the catalyst is a relatively coarse granular material. Recovery of catalyst metal values from suspended fines in a hydrgenated product is usually accomplished by separation and chemical treatment of the fines. Johnson, U.S. Pat. No. 3,679,573 and Weber et al., U.S. Pat. No. 3,622,265 describe ebullated bed processes wherein catalyst particles are settled out of a reaction mixture and returned to the process.
These catalytic hydrogenations of coal are usually run at about 400.degree. C-500.degree. C and with about 1000-5000 psi hydrogen pressure. Crushed or pulverized coal is dispersed in a hydrocarbon oil, usually oil recycled from the process effluent, to make a feed slurry of 10-50 percent coal content.
In noncatalytic coal hydrogenation processes, the hydrogenation is facilitated by the large surface provided by relatively fine particles of ash in the coal and probably also by the metal content of the ash, primarily iron. Noncatalytic processes have been described where this surface effect is expanded by addition of more such solid particles in order to improve the relatively low conversion typical of this kind of process. For example, Wolk et al., U.S. Pat. No. 3,617,465 discloses a process wherein the ash content of an effluent product is concentrated by a preliminary distillation and at least a part of the ash-containing bottoms is recycled to the hydrogenation reactor.